network engineering, system administration, security tools

Logs are useful when you want to track usage or troubleshoot an application. As more information gets logged, however, log files use more disk space. Over time a log file can grow to unwieldy size. Running out of disk space because of a large log file is a problem, but a large log file can also slow down the process of resizing or backing up your virtual server. Additionally, it’s hard to look for a particular event if you have a million log entries to skim through. So it’s a good idea to keep log files down to a manageable size, and to prune them when they get too old to be of much use.

Fortunately, the logrotate utility makes log rotation easy. “Log rotation” refers to the practice of archiving an application’s current log, starting a fresh log, and deleting older logs. The system usually runs logrotate once a day, and when it runs it checks rules that can be customized on a per-directory or per-log basis.

How logrotate works

The system runs logrotate on a schedule, usually daily. On most distributions, the script that runs logrotate daily is located at /etc/cron.daily/logrotate.

Some distributions use a variation. For example, on Gentoo the logrotate script is located at /etc/cron.daily/logrotate.cron.

If you want logrotate to run more often (for hourly log rotation, for example) you need to use cron to run logrotate through a script in /etc/cron.hourly.

When logrotate runs, it reads its configuration files to determine where to find the log files that it needs to rotate, how often the files should be rotated, and how many archived logs to keep.

logrotate.conf

The main logrotate configuration file is located at /etc/logrotate.conf.

The file contains the default parameters that logrotate uses when it rotates logs. The file is commented, so you can skim it to see how the configuration is set up. Several of the specific commands in that file are described later in this article.

Note that one line in the file reads:

include /etc/logrotate.d

That directory contains most of the application-specific configuration files.

logrotate.d

Use the following command to list contents of the directory that stores application-specific log settings:

ls /etc/logrotate.d

Depending on how much is installed on your server, this directory might contain no files or several. In general, applications that are installed through your package manager will also create a config file in /etc/logrotate.d.

Usually the directory contains a configuration file for your syslog service, which logrotate reads when it rotates the system logs. This file contains an entry for various system logs, along with some commands similar to those contained in logrotate.conf.

NOTE: On versions of Ubuntu earlier than Karmic Koala (9.10) there is no entry for a syslog service. Before that release, the system logs were rotated by a savelog command run from the /etc/cron.daily/sysklogd script.

Inside an application file

As an example, consider the contents of a logrotate configuration file that might be put in place when you install Apache on a Fedora system:

When logrotate runs, it checks for any files in /var/log/httpd that end in log and rotates them, if they aren’t empty. If it checks the httpd directory and doesn’t find any log files, it doesn’t generate an error. Then it runs the command in the postrotate/endscript block (in this case, a command that tells Apache to restart), but only after it has processed all the specified logs.

This example file does not contain some settings that are included in the logrotate.conf file. The commands in logrotate.conf act as defaults for log rotation. You can specify different settings for any application when you want to override the defaults. For example, if you run a busy web server, you might want to include a daily command in Apache’s configuration block so that Apache’s logs will rotate daily instead of the default weekly rotation.

The next section describes some of the more commonly-used commands actually do in a logrotate configuration file.

Configuration commands

You can get a full list of commands used in logrotate configuration files by checking the man page:

man logrotate

This section describes the more commonly-used commands.

Remember, the configuration files for applications in /etc/logrotate.d inherit their defaults from the main /etc/logrotate.conf file.

Log files

A log file and its rotation behavior are defined by listing the log file (or files) followed by a set of commands enclosed in curly brackets. Most application configuration files will contain just one of these blocks, but it’s possible to put more than one in a file, or to add log file blocks to the main logrotate.conf file.

You can list more than one log file for a block by using a wildcard in the name or by separating log files in the list with spaces. For example, to specify all files in the directory /var/foo that end in .log, and the file /var/bar/log.txt, you would set up the block as follows:

Rotate count

The rotate command determines how many archived logs are returned before logrotate starts deleting the older ones. For example:

rotate 4

This command tells logrotate to keep four archived logs at a time. If four archived logs exist when the log is rotated again, the oldest one is deleted to make room for the new archive.

Rotation interval

You can specify a command that tells logrotate how often to rotate a particular log. The possible commands include:

daily
weekly
monthly
yearly

If a rotation interval is not specified the log will be rotated whenever logrotate runs (unless another condition like size has been set).

If you want to use a time interval other than the the defined ones, you need to use cron to create a separate configuration file. For example, if you want to rotate a particular log file hourly, you could create a file in /etc/cron.hourly (you might need to create that directory too) that would contain a line like the following:

/usr/sbin/logrotate /etc/logrotate.hourly.conf

Then you would put the configuration for that hourly run of logrotate (the log file location, whether or not to compress old files, and so on) into /etc/logrotate.hourly.conf.

Size

You can use the size command to specify a file size for logrotate to check when determining whether to perform a rotation. The format of the command tells logrotate what units you’re using to specify the size:

size 100k
size 100M
size 100G

The first example would rotate the log if it gets larger than 100 kilobytes, and the second if it’s larger than 100 megabytes, and the third if it’s over 100 gigabytes. I don’t recommend using a limit of 100G, mind you, the example just got a little out of hand there.

The size command takes priority over and replaces a rotation interval if both are set.

Compression

If you want archived log files to be compressed (in gzip format), you can include the following command, usually in /etc/logrotate.conf:

compress

Compression is normally a good idea, because log files are usually all text and text compresses well. If, however, you have some archived logs that you don’t want to compress, but you still want compression to be on by default, you can include the following command in an application-specific configuration:

nocompress

Another command of note in regard to compression is as follows:

delaycompress

This command is useful if you want to compress the archived logs, but want to delay the compression. When delaycompress is active, an archived log is compressed the next time that the log is rotated. This can be important when you have a program that might still write to its old log file for a time after a fresh one is rotated in. Note that delaycompress works only if you have compress in your configuration.

An example of a good time to use delaycompress would be when logrotate is told to restart Apache with the “graceful” or “reload” directive. Because old Apache processes do not end until their connections are finished, they could potentially try to log more items to the old file for some time after the restart. Delaying the compression ensures that you won’t lose those extra log entries when the logs are rotated.

Postrotate

Logrotate runs the postrotate script each time it rotates a log specified in a configuration block. You usually want to use this script to restart an application after the log rotation so that the app can switch to a new log.

postrotate
/usr/sbin/apachectl restart > /dev/null
endscript

>/dev/null tells logrotate to pipe the command’s output to nowhere. In this case, you don’t need to view the the output if the application restarted correctly.

The postrotate command tells logrotate that the script to run, starts on the next line, and the endscript command says that the script is done.

Sharedscripts

Normally logrotate runs the postrotate script every time it rotates a log. This is also true for multiple logs that use the same configuration block. For example, a web server configuration block that refers to both the access log and the error log will, if it rotates both, run the postrotate script twice (once for each file rotated). If both files are rotated, the web server is restarted twice.

To keep logrotate from running that script for every log, you can include the following command:

sharedscripts

This command tells logrotate to check all the logs for that configuration block before running the postrotate script. If one or both of the logs is rotated, the postrotate script runs only once. If none of the logs is rotated, the postrotate script doesn’t run.Source